The present invention concerns a catalytic alkylation process for at least one isoparaffin selected from the group formed by isobutane and isopentane by at least one olefin containing 2 to 6, preferably 3 to 6, carbon atoms per molecule, in the presence of at least one liquid acid catalyst.
The alkylation of an isoparaffin (isobutane and/or isopentane) by at least one olefin containing 2 to 6, preferably 3 to 6, carbon atoms per molecule can produce highly branched paraffinic hydrocarbons (for example from the group constituted by dimethylbutanes, trimethylpentanes, trimethylhexanes and trimethylheptanes), which are essential constituents of high octane number fuels. This alkylation reaction, termed aliphatic alkylation, requires the use of highly acidic catalysts primarily to reduce side reactions such as hydride abstraction from the olefin and polymerisation which produce less highly branched hydrocarbons with low octane numbers and unsaturated hydrocarbons, also cracking reactions and dismutation reactions.
A large number of liquid and solid acid catalysts are known for aliphatic alkylation of isoparaffins such as isobutane or isopentane, by at least one olefin such as propylene, 1- and 2-butenes, or isobutene. Currently, the most frequently used catalysts in the industry are liquid catalysts, namely concentrated sulphuric acid and hydrofluoric acid, used alone or as a mixture with Lewis acids such as boron trifluoride.
The majority of conventional processes are characterized in that the acid phase constitutes the continuous phase of the acid-hydrocarbon emulsion formed in the reactor.
The volume ratio of acid to hydrocarbons (mixture of isoparaffins and olefins) is thus greater than one. Further, the emulsion is generally formed in the alkylation reactor itself, i.e., in the presence of the feed to be converted which contains the olefin (see, for example, the use of Stratco technology employing liquid sulphuric acid: L. F. Albright, Chem. Eng., Aug. 15, 1966, p. 143 and L. F. Albright, Oil & Gas Journal, Nov. 12, 1990).
When the catalyst is sulphuric acid, the conventional isoparaffin alkylation process using a sulphuric acid catalyst has a number of disadvantages, among them: the impossibility of reaching temperatures of less than 0.degree. C. in the reactor in the presence of more than 97% by weight strength sulphuric acid. The viscosity of sulphuric acid becomes too high at such temperatures and, since the acid is the continuous phase, it becomes impossible to stir the medium; the olefin in the feed and the acid recycled from the settler are injected directly into the reactor in immediate proximity to the moving part of the stirrer. Thus this latter must carry out two operations: create the acid-hydrocarbon emulsion and dilute the olefin in the hydrocarbon phase to limit local superconcentration of the olefin, which is not carried out sufficiently effectively; the residence time of the hydrocarbon phase in the settler is high, usually about one hour. Since the temperature in the settler is usually above about 15.degree. C., some isoparaffins containing more than 5 carbon atoms per molecule undergo degradation reactions. Examples of these degradation reactions are oxidation of paraffins by sulphuric acid to produce water and SO.sub.2. These reactions contribute to catalyst deactivation. Other reactions are uncontrolled decomposition of alkyl sulphates, in particular butyl sulphates. These decomposition reactions are the cause of the formation of unsaturated oligomers which deactivate the catalyst.
International patent application PCT WO 95/04.019 describes a process for the alkylation of an olefin by an isoparaffin in the presence of liquid sulphuric acid, the process comprising a zone for preparing an emulsion of acid in the isoparaffin followed by a reaction zone supplied with the emulsion into which the olefin is injected, the ratio by volume of sulphuric acid: hydrocarbons present in the reaction zone being in the range 0.3:1 to 0.5:1. Thus the continuous phase of the prepared emulsion in that process is the hydrocarbon phase. That patent application also describes means for forming the emulsion and adding the olefin to the reaction zone, as well as an apparatus for carrying out the process.
In International patent application PCT WO 95/04.019, the olefin is injected directly into the reaction zone. Thus in that zone, the olefin must be diluted in the hydrocarbon phase in such a manner as to limit local superconcentration of the olefin.
The alkylation of isobutane and/or isopentane comprises a first, very rapid, step of adsorption of the olefin on the acid catalyst to form a type of acid-olefin "complex", followed by reaction of the "complex" with isobutane and/or isopentane. If carried out under the temperature and pressure conditions used for the reaction, the first step can encourage the formation of polymers, which are undesirable secondary products, to the detriment of "complex" formation. This has a highly deleterious effect on the process yield. It is thus of advantage to mix the olefin and the catalyst in a zone preceding the reaction zone, under temperature and pressure conditions which are primarily favourable to formation of the "complex".